Switching power supply

ABSTRACT

A switching power supply has first and second switching elements that are alternately turned on to cause an alternating current to flow through a primary winding of a transformer. The alternating current flowing through the primary winding induces voltages respectively across first and second secondary windings of the transformer. Synchronous rectifying transistors are turned on by the voltages induced across the first and second secondary windings, causing a current to flow alternately through the first and second secondary windings. Either one of the first and second switching elements is turned on while the other is being turned off, except for short off-times in which both the first and second switching elements are prevented from being turned on. As a result, when currents flow through the first and second secondary windings, the synchronous rectifying transistors are turned on. Therefore, no current flows through internal parasitic diodes of the synchronous rectifying transistors, and hence any loss caused thereby is very small.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a switching power supply which is highly efficient in operation.

2. Description of the Related Art

FIGS. 1 and 2 of the accompanying drawings show conventional switching power supplies. The conventional switching power supply shown in FIG. 1 is of the general forward type. For making a highly efficient switching power supply, particularly a switching power supply with a low-voltage (e.g., 5 V or 3.3 V), high-current output capability, using the circuit shown in FIG. 1, since rectifying diodes D₁₀₁, D₁₀₂ connected to the secondary winding N₁₂ of a transformer T₁₀₁ cause a large power loss, it is often customary to employ synchronous rectifying MOSFETs in place of the rectifying diodes D₁₀₁, D₁₀₂. It is of importance to consider how these synchronous rectifying MOSFETs (also referred to as synchronous rectifying transistors) are to be driven to achieve a highly efficient switching power supply.

The conventional switching power supply shown in FIG. 2 employs synchronous rectifying transistors Q₁₀₂, Q₁₀₃ instead of the rectifying diodes.

The conventional switching power supply shown in FIG. 1 which employs the rectifying diodes suffers the same problems as those of the conventional switching power supply shown in FIG. 2 which employs the synchronous rectifying transistors, except that the synchronous rectifying transistors are driven in the conventional switching power supply shown in FIG. 2. Therefore, the conventional switching power supply shown in FIG. 2 will be described below.

FIG. 3 of the accompanying drawings shows the waveforms of voltages and currents in various parts of the conventional switching power supply shown in FIG. 2. FIG. 4 of the accompanying drawings shows an output voltage of the conventional switching power supply shown in FIG. 2 with respect to a duty cycle (the ratio of an on-time to an operating period of a switching element Q₁₀₁) thereof.

In FIG. 3, T₁ represents an operating period of the switching element Q₁₀₁, T_(ON1) represents an on-time thereof, and T_(off1), T_(off2) represent an off-time thereof.

V_(gs) (Q₁₀₁), I_(d) (Q₁₀₁), and V_(ds) (Q₁₀₁) represent agate voltage, a drain current, and a drain-to-source voltage, respectively, of the switching element Q₁₀₁, and V(N₁₁) represents a voltage across the primary winding N₁₁ of the transformer T₁₀₁.

Of the voltage V(N₁₁) across the primary winding of the transformer T₁₀₁, a voltage V(₁) in the off-time T_(off1) of the switching element Q₁₀₁ is generated to reset the transformer T₁₀₁, after it has been excited in the on-time T_(ON1) of the switching element Q₁₀₁. The voltage V(h₁) is generated such that an integral of the voltage with respect to time in the on-time will be equal to an integral of the voltage with respect to time in the off-time.

The waveform of the voltage V(N₁₁) in the off-time T_(off1) is determined depending on the magnetizing inductance and the capacitance between output terminals of the switching element Q₁₀₁. When the voltage of an input power supply V_(in) and an output current supplied to a load vary, the duty cycle of the switching element Q₁₀₁ varies to keep the output voltage constant, and the voltage V(h₁) and the off-times T_(off1), T_(off2) also vary.

In order to reset the transformer T₁₀₁ in the off-time T_(off1) by exactly the same quantity as it has been excited in the on-time T_(ON1) under any input and output conditions, it is necessary to sufficiently provide the off-time T_(off2) in which no voltage is induced across the primary winding N₁₁, after the resetting of the transformer T₁₀₁. As described later on, the need to increase the off-time T_(off2) poses a serious problem.

In FIG. 3, V_(ds) (Q₁₀₂) represents a drain-to-source voltage of the synchronous rectifying transistor Q₁₀₂ , and V_(ds) (Q₁₀₃) represents a drain-to-source voltage of the synchronous rectifying transistor Q₁₀₃. These voltages are voltages converted from the voltage V(N₁₁), in the respective off-and on-times T_(off1), T_(ON1), across the primary winding N₁₁ of the transformer T₁₀₁ with the turns ratio of the primary and secondary windings N₁₁, N₁₂ of the transformer T₁₀₁.

One major problem encountered in making a highly efficient switching power supply using the circuit shown in FIG. 2 is that since the drain-to-source voltage V_(ds) (Q₁₀₂) of the synchronous rectifying transistor Q₁₀₂ is large as shown in FIG. 3, the synchronous rectifying transistor Q₁₀₂ has a large on-state resistance and causes a large power loss, resulting in a reduction in the switching power supply efficiency.

Specifically, though the synchronous rectifying transistor Q₁₀₂ should have a smaller on-state resistance for higher switching power supply efficiency, MOSFETs have such a general tendency that their on-state resistance is higher as the drain-source breakdown voltage is higher.

The voltage V(N₁₁) in the off-time T_(off1) across the primary winding N₁₁ of the transformer T₁₀₁ is of a sine wave because it resonates with the magnetizing inductance and the capacitance between the drain and source of the switching element Q₁₀₁, and hence has a large maximum level. Furthermore, since the voltage V(N₁₁) varies greatly depending on the input and output conditions, the synchronous rectifying transistor Q₁₀₂ is required to have a large dielectric strength between the drain and source thereof and hence a large on-state resistance.

The above problem holds true for the conventional switching power supply shown in FIG. 1 where the rectifying diode D₁₀₁, is used in place of the synchronous rectifying transistor Q₁₀₂.

The problem of the dielectric strength of the synchronous rectifying transistor Q₁₀₂ is also the problem of the dielectric strength of the switching element Q₁₀₁. One conventional way of limiting the dielectric strength to a certain voltage is to use a clamping circuit, which comprises a diode, a capacitor, and a resistor, between the terminals of the primary winding N₁₁ of the transformer T₁₀₁. Though the clamping circuit is capable of clamping the voltage to a certain level, however, the efficiency is lowered because the magnetization energy of the transformer T₁₀₁ is consumed by the resistance of the clamping circuit. According to another conventional way of clamping the voltage, a tertiary winding is added to the transformer T₁₀₁, and connected to the input power supply V_(in) via a diode, thus providing a clamping circuit.

With the latter conventional clamping circuit, most of the excitation energy of the transformer T₁₀₁ flows to the input power supply V_(in). When this current flows through the diode of the clamping circuit, the diode develops a voltage drop which causes a power consumption resulting in a reduction in the efficiency. In addition, the transformer T₁₀₁ is large in size and complex in structure because of the added tertiary winding, and the tertiary winding suffers an increased conduction loss.

Another drawback which results from making a highly efficient switching power supply using the circuit shown in FIG. 2 is that because of the off-time T_(off2) shown in FIG. 3, the synchronous rectifying transistor Q₁₀₃ cannot be energized for the entire period in which the switching element Q₁₀₁ is turned off, resulting in a reduction in the switching power supply efficiency. This drawback is inherent in using synchronous rectifying transistors, and is the most serious in the manufacture of highly efficient switching power supplies. In the on-time T_(ON1) of the switching element Q₁₀₁, the synchronous rectifying transistor Q₁₀₃ is turned off and the synchronous rectifying transistor Q₁₀₂ is turned on, and a current through a choke coil L₁₀ flows through the synchronous rectifying transistor Q₁₀₂. In the off-time T_(off1) of the switching element Q₁₀₁, the synchronous rectifying transistor Q₁₀₂ is turned off and the synchronous rectifying transistor Q₁₀₃ is turned on, and a current through a choke coil L₁₀ flows through the synchronous rectifying transistor Q₁₀₃.

The gate terminals of the synchronous rectifying transistors Q₁₀₂, Q₁₀₃ are energized by the voltage across the secondary winding N₁₂ of the transformer T₁₀₁. When one of the synchronous rectifying transistors Q₁₀₂, Q₁₀₃ is turned on, the gate voltage thereof comes from the drain-to-source voltage of the other synchronous rectifying transistor which is turned off.

The switching element Q₁₀₁ has the off-times T_(off1), T_(off2). As can be seen from FIG. 3, since the drain-to-source voltage V_(ds) (Q₁₀₂) of the synchronous rectifying transistor Q₁₀₂ has a certain level in the off-time T_(off1), the synchronous rectifying transistor Q₁₀₃ can be energized in the off-time T_(off1). However, in the off-time T_(off2), the synchronous rectifying transistor Q103 cannot be energized because the drain-to-source voltage V_(ds) (Q₁₀₂) is nil. Therefore, the synchronous rectifying transistor Q₁₀₃ is turned off in the off-time T_(off2). During this time, a current through the choke coil L₁₀ flows through a body diode of the synchronous rectifying transistor Q₁₀₃, i.e., a parasitic diode inserted from the source terminal to the drain terminal thereof because of the MOSFET structure. Inasmuch as a voltage drop across the body diode is much greater than a voltage drop caused when the synchronous rectifying transistor Q₁₀₃ is turned on, the power loss in the off-time T_(off2) is increased, reducing the switching power supply efficiency.

Still another shortcoming is that the choke coil of the output filter is large because an output ripple voltage is high. Stated otherwise, if a chock coil of a certain size is used to keep the output ripple voltage to a prescribed level, then the iron loss and copper loss of the chock coil are increased, indirectly lowering the efficiency of the switching power supply.

In FIG. 3, the drain-to-source voltage V_(ds) (Q₁₀₃) of the synchronous rectifying transistor Q₁₀₃ is the same as a voltage V(P) at a point P shown in FIG. 2. The voltage V(P) is averaged by an output filter, which comprises the choke coil L₁₀ and a capacitor C₁₀, into an output voltage V_(out) free of alternating current components. The output voltage V_(out) is indicated in relation to the drain-to-source voltage V_(ds) (Q₁₀₃) in FIG. 3. The drain-to-source voltage V_(ds) (Q₁₀₃) and the output voltage V_(out) across the capacitor C₁₀ are applied respectively to the terminals of the choke coil L₁₀. The difference between the drain-to-source voltage V_(ds) (Q₁₀₃) and the output voltage V_(out) determines a ripple current I(L₁₀) flowing through the choke coil L₁₀. The product of the ripple current I(L₁₀) and the equivalent series resistance of the capacitor C₁₀ approximately determines the value of an output ripple voltage.

FIG. 4 shows an output voltage of the conventional switching power supply shown in FIG. 2 with respect to a duty cycle thereof, as described above. As can be understood from FIG. 4, since the output voltage is proportional to the duty cycle, the duty cycle is set to nearly 0.5 generally when the input and output conditions are rated conditions. According to the waveform of the drain-to-source voltage V_(ds) (Q₁₀₃), the ratio of the period in which the drain-to-source voltage V_(ds) (Q₁₀₃) is nil, i.e., the sum of the off-times T_(off1), T_(off2), to the entire time, i.e., the operating period T₁, is about 50%, so that the output ripple voltage is large. Stated otherwise, if a chock coil of a certain size is used to keep the output ripple voltage to a prescribed level, then the iron loss and copper loss of the chock coil are increased, indirectly lowering the efficiency of the switching power supply.

As described above, when a highly efficient switching power supply is to be constructed using the circuit shown in FIG. 2, the rectifying elements of the rectifying circuit connected to the secondary winding of the transformer cause a large power loss, and it is important to consider how the power loss can be reduced. Recently, efforts have been made to use synchronous rectifying transistors in place of the rectifying elements for there by reducing their conduction loss. It is also important to pay attention to effective energization of the synchronous rectifying transistors.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a switching power supply which employs rectifying elements of low dielectric strength that suffer a relatively small power loss, and allows synchronous rectifying transistors used as such rectifying elements to be energized for an entire period, so that the switching power supply can operate highly efficiently.

According to an aspect of the present invention, there is provided a power supply circuit comprising first and second choke coils, first and second capacitors, first and second switching elements each comprising a MOSFET, first and second synchronous rectifying transistors each comprising a MOSFET, a transformer having a primary winding and first and second secondary windings which are magnetically coupled to the primary winding, the first and second switching elements being connected in series with each other at a junction, jointly making up a series-connected circuit, the primary winding and the first choke coil having respective terminals connected to the junction between the first and second switching elements, the first and second capacitors being connected in series with each other at a junction, jointly making up a series-connected circuit, the primary winding having another terminal connected to the junction between the first and second capacitors, the series-connected circuit of the first and second switching elements and the series-connected circuit of the first and second capacitors being connected parallel to each other, jointly making up a parallel-connected circuit, and a power supply for applying a voltage between a terminal of the parallel-connected circuit and another terminal of the first choke coil, the arrangement being such that when the first and second switching elements are alternately turned on, an alternating current flows through the primary winding to induce alternating-current voltages respectively across the first and second secondary windings for thereby alternately turning on the first and second synchronous rectifying transistors to cause a current to flow alternatively through the first and second secondary windings for supplying a current to the second choke coil.

According to another aspect of the present invention, there is also provided a power supply circuit comprising first and second choke coils, first and second capacitors, first and second switching elements each comprising a MOSFET, first and second synchronous rectifying transistors each comprising a MOSFET, a transformer having a primary winding and first and second secondary windings which are magnetically coupled to the primary winding, the first and second switching elements being connected in series with each other at a junction, jointly making up a series-connected circuit, the primary winding and the first choke coil having respective terminals connected to the junction between the first and second switching elements, the second capacitor being connected parallel to the series-connected circuit of the first and second switching elements, jointly making up a parallel-connected circuit, the parallel-connected circuit having a terminal connected by the first capacitor to another terminal of the primary winding, and a power supply for applying a voltage between a junction between the parallel-connected circuit and the first capacitor, and another terminal of the first choke coil, the arrangement being such that when the first and second switching elements are alternately turned on, an alternating current flows through the primary winding to induce alternating-current voltages respectively across the first and second secondary windings for thereby alternately turning on the first and second synchronous rectifying transistors to cause a current to flow alternatively through the first and second secondary windings for supplying a current to the second choke coil.

According to still another aspect of the present invention, there is further provided a power supply circuit comprising first and second choke coils, first and second capacitors, first and second switching elements each comprising a MOSFET, first and second synchronous rectifying transistors each comprising a MOSFET, a transformer having a primary winding, first and second secondary windings which are magnetically coupled to the primary winding, and a tertiary winding which is magnetically coupled to the primary winding and the first and second secondary windings, the primary winding and the first capacitor being connected in series with each other, jointly making up a series-connected circuit, the first switching element being connected parallel to the series-connected circuit of the primary winding and the first capacitor, jointly making up a parallel-connected circuit, the parallel-connected circuit having a terminal connected to a terminal of the first choke coil, and a power supply for applying a voltage between another terminal of the parallel-connected circuit and another terminal of the first choke coil, the tertiary winding and the second capacitor being connected in series with each other, jointly making up a series-connected circuit, the second switching element being connected parallel to the series-connected circuit of the tertiary winding and the second capacitor, the arrangement being such that when the first and second switching elements are alternately turned on, an alternating current flows through the primary winding to induce alternating-current voltages respectively across the first and second secondary windings for thereby alternately turning on the first and second synchronous rectifying transistors to cause a current to flow alternatively through the first and second secondary windings for supplying a current to the second choke coil.

In each of the above power supply circuits, the first and second secondary windings have respective terminals connected to each other at a junction, the second choke coil having a terminal connected to the junction between the first and second secondary windings, and the first and second secondary windings have respective other terminals connected to each other by the first or second synchronous rectifying transistor at a junction, with an output voltage being extracted between the junction between the other terminals of the first and second secondary windings and another terminal of the second choke coil, the arrangement being such that a voltage at the other terminal of the second secondary winding is applied to a gate terminal of one of the first and second synchronous rectifying transistors which is connected to the first secondary winding, and a voltage at the other terminal of the first secondary winding is applied to a gate terminal of one of the first and second synchronous rectifying transistors which is connected to the second secondary winding.

Either one of the first and second switching elements is turned on for an on-time except for a relatively short off-time in which both of the first and second switching elements are prevented from being turned on at the same time, the on-time being variable to keep constant the output voltage extracted from the second choke coil.

The power supply circuit further comprises an output capacitor for smoothing the output voltage.

The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a conventional switching power supply;

FIG. 2 is a circuit diagram of another conventional switching power supply;

FIG. 3 is a diagram showing the waveforms of voltages and currents in various parts of the conventional switching power supply shown in FIG. 2;

FIG. 4 is a diagram showing an output voltage of the conventional switching power supply shown in FIG. 2 with respect to a duty cycle (the ratio of on-time to operating period of a switching element Q₁₀₁) thereof;

FIG. 5 is a circuit diagram of a switching power supply according to a first embodiment of the present invention;

FIG. 6 is a circuit diagram showing a current path in the switching power supply according to the first embodiment of the present invention;

FIG. 7 is a circuit diagram showing another current path in the switching power supply according to the first embodiment of the present invention;

FIG. 8 is a circuit diagram showing other current paths in the switching power supply according to the first embodiment of the present invention;

FIG. 9 is a circuit diagram showing other current paths in the switching power supply according to the first embodiment of the present invention;

FIG. 10 is a diagram showing the waveforms of voltages and currents in various parts of the switching power supply according to the first embodiment of the present invention;

FIG. 11 is a diagram showing an output voltage of the switching power supply according to the first embodiment of the present invention with respect to a duty cycle (the ratio of on-time to operating period of a switching element Q₁) thereof;

FIGS. 12a through 12c are circuit diagrams illustrative of the manner in which the switching power supply according to the first embodiment of the present invention operates;

FIG. 13 is a circuit diagram illustrative of another process of driving synchronous rectifying MOSFETs;

FIG. 14 is a circuit diagram of a switching power supply according to a second embodiment of the present invention;

FIGS. 15a and 15b are circuit diagrams illustrative of the manner in which the switching power supply according to the second embodiment of the present invention operates;

FIG. 16 is a circuit diagram of a switching power supply according to a third embodiment of the present invention;

FIG. 17 is a circuit diagram showing a current path in the switching power supply according to the third embodiment of the present invention;

FIG. 18 is a circuit diagram showing current paths in the switching power supply according to the third embodiment of the present invention; and

FIG. 19 is a circuit diagram showing current paths in the switching power supply according to the third embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 5, a switching power supply according to a first embodiment of the present invention has an input power supply V_(in), a pair of input terminals 2_(a), 2_(b), a first choke coil L₁, a pair of first and second switching elements Q₁, Q₂, a pair of first and second capacitors C₁, C₂, a transformer T having a primary winding N₁ and a pair of first and second secondary windings N_(a), N_(b), a pair of first and second synchronous rectifying transistors Q₃, Q₄, an output filter comprising a second choke coil L₂ and an output capacitor C_(out), a pair of output terminals 16_(a), 16_(b), a load 17, and a control circuit 18.

The parts connected to the primary winding N₁ of the transformer T will be described in detail below.

The input power supply V_(in) has a positive terminal connected to the input terminal 2_(a) and a negative terminal connected to the input terminal 2_(b) which is kept at ground potential.

Each of the first and second switching elements Q₁, Q₂ comprises an n-channel MOSFET. The first switching element Q₁ has a drain terminal connected to the source terminal of the second switching element Q₂ and a source terminal connected to ground. The first and second switching elements Q₁, Q₂ jointly make up a series-connected circuit 41. Diodes which are shown connected respectively across the first and second switching elements Q₁, Q₂ are parasitic diodes in their MOSFETS.

The first and second capacitors C₁, C₂ are connected in series with other and jointly make up a series-connected circuit 42 which has a terminal connected to the drain terminal of the second switching element Q₂ and an opposite terminal connected to ground. The series-connected circuit 42 is connected parallel to the series-connected circuit 41, jointly making up a parallel-connected circuit 43.

The first choke coil L₁ has a terminal connected to the input terminal 2a connected to the positive terminal of the input power supply V_(in). The other terminal of the first choke coil L₁ is connected to the junction between the first and second switching elements Q₁, Q₂.

The junction between the first and second switching elements Q₁, Q₂ is connected to a terminal of the primary winding N₁ of the transformer T. The other terminal of the primary winding N₁ is connected to the junction between the first and second capacitors C₁, C₂.

The parts connected to the first and second secondary windings N_(a), N_(b) of the transformer T will be described in detail below. Each of the first and second synchronous rectifying transistors Q₃, Q₄ comprises an n-channel MOSFET. The first and second secondary windings N_(a), N_(b) have terminals connected to each other and other terminals connected to the gate terminals of the first and second synchronous rectifying transistors Q₃, Q₄. The junction between the first secondary winding N_(a) and the gate terminal of the first synchronous rectifying transistor Q₃ is connected to the drain terminal of the second synchronous rectifying transistor Q₄. Similarly, the junction between the second secondary winding N_(b) and the gate terminal of the second synchronous rectifying transistor Q₄ is connected to the drain terminal of the first synchronous rectifying transistor Q₃.

The first and second synchronous rectifying transistors Q₃, Q₄ have respective source terminals connected to each other, and the output terminal 16_(b) which is held at ground potential is connected to the junction between the source terminals of the first and second synchronous rectifying transistors Q₃, Q₄.

The first and second secondary windings N_(a), N_(b) are magnetically coupled to the primary winding N₁ of the transformer T. The terminal of the primary winding N₁ which is connected to the first and second switching elements Q₁, Q₂ is of the same polarity as that of the terminal of the first secondary winding N_(a) which is connected to the second synchronous rectifying transistor Q₄, and also of the same polarity as that of the terminal of the second secondary winding N_(b) which is connected to the first secondary winding N_(a).

The junction between the first and second secondary windings N_(a), N_(b) is connected to a terminal of the second choke coil L₂, whose other terminal is connected to the output terminal 16_(a) which is kept at a positive potential.

The output capacitor C_(out) is connected between the output terminals 16_(a), 16_(b). The load 17 is also connected between the output terminals 16_(a), 16_(b) parallel to the output capacitor C_(out).

After a voltage from the input power supply V_(in) is applied, when the first switching element Q₁ is turned on with the second switching element Q₂ being turned off, a current flows along a path indicated by J₁ in FIG. 6, storing energy in the first choke coil L₁.

Then, when the first switching element Q₁ is turned off and the second switching element Q₂ is turned on, an electromotive force is induced across the choke coil L₁ by the energy stored in the choke coil L₁, rendering the second switching element Q₂ conductive in the reverse direction. A current flows in a path indicated by J₂ in FIG. 7 from the source terminal to the drain terminal of the second switching element Q₂, charging the first and second capacitors C₁, C₂.

The state in which the first and second capacitors C₁, C₂ are charged is referred to as a steady state. When the first switching element Q₁ is turned off and the second switching element Q₂ is turned on in the steady state, the second capacitor C₂ is discharged, causing a current to flow from the drain terminal of the second switching element Q₂ to the source terminal thereof to the primary winding N₁ along a path indicated by J₃ in FIG. 8.

When the first switching element Q₁ is turned on and the second switching element Q₂ is turned off in the steady state, the first capacitor C₁ is discharged, causing a current to flow from the drain terminal of the first switching element Q₁ to the source terminal thereof to the primary winding N₁ along a path indicated by J₄ in FIG. 9. At this time, the current flows through the primary winding N₁ in the direction opposite to the direction in which it flows when the second switching element Q₂ is turned on.

Operation of the switching power supply shown in FIG. 5 in the steady state will be described below with reference to FIG. 10.

In FIG. 10, T₃₁ represents an operating period of the switching elements, T_(ON31) represents an on-time of the first switching element Q₁, T_(ON32) represents an on-time of the second switching element Q₂, and T_(off31), T_(off32) represent off-times in which both the first and second switching elements Q₁, Q₂ are turned off. The off-times T_(off31), T_(off32) are a period in which both the first and second switching elements Q₁, Q₂ are prevented from being turned on at the same time, so that series-connected circuit of the first and second capacitors C₁, C₂ is prevented from being short-circuited. The off-times T_(off31), T_(off32) may be of a minimum required period in view of delay times of the first and second switching elements Q₁, Q₂ upon switching thereof.

In FIG. 10, V_(gs) (Q₁), V_(gs) (Q₂) represent gate drive voltages of the first and second switching elements Q₁, Q₂, respectively.

It can be seen from FIG. 10 that the first and second switching elements Q₁, Q₂ are controlled by the control circuit 18 such that when one of the first and second switching elements Q₁, Q₂ is turned on, the other is turned off, and vice versa except for the short off-times T_(off31), T_(off32), and that the ratio of the on-time of one of the first and second switching elements Q₁, Q₂ to the operating period T₃₁, i.e., the duty cycle, is varied to produce a constant output voltage V_(out).

In FIG. 10, I(L₁) represents a current flowing through the first choke coil L₁, I(N₁) and V(N₁) represent a current flowing through and a voltage developed across the primary winding N₁ of the transformer T, respectively, I(Q₁) and I(Q₂) represent currents flowing through the first and second switching elements Q₁, Q₂, respectively, V_(ds) (Q₃), V_(ds) (Q₄) represent drain-to-source voltages of the first and second synchronous rectifying transistors Q₃, Q₄, respectively, V(R) represents a voltage at the junction (point R in FIG. 5) between the first and second secondary windings N_(a), N_(b) of the transformer T, and I(L₂) represents a current flowing through the second choke coil L₂.

Prior to describing the waveforms of voltages and currents shown in FIG. 10, operation in the steady state of the switching power supply shown in FIG. 5 will be described below with reference to FIGS. 12a, 12b, and 12c.

Operation of the switching power supply shown in FIG. 5 may be considered as a combination of operation of the circuits shown in FIGS. 12a, 12b. In the switching power supply shown in FIG. 5, the second switching element Q₂ is turned off in the on-time of the first switching element Q₁, and turned on in the off-time of the first switching element Q₁. Therefore, the second switching element Q₂ operates in the same manner as a commutating diode D₂₁ in a booster chopper circuit shown in FIG. 12c. Therefore, the circuit shown in FIG. 12a may be replaced with the circuit shown in FIG. 12c, and hence may be regarded as a booster chopper circuit.

Since the series-connected circuit of the first and second capacitors C₁, C₂ shown in FIG. 12b is charged to the single polarity as described above with reference to FIG. 7, it is possible for the first and second capacitors C₁, C₂ to maintain a certain DC voltage if the first and second capacitors C₁, C₂ have a sufficiently large capacitance. Consequently, the series-connected circuit of the first and second switching elements Q₁, Q₂ may be considered as a half-bridge circuit with the series-connected circuit of the first and second capacitors C₁, C₂ serving as its input power supply.

The operation of the switching power supply shown in FIG. 5 may be considered as a combination of the operation of the booster chopper circuit shown in FIG. 12a in which the input power supply V_(in) supplies electric energy to the series-connected circuit of the first and second capacitors C₁, C₂ and the operation of the half-bridge circuit shown in FIG. 12b in which the series-connected circuit of the first and second capacitors C₁, C₂ supplies electric energy to the load 17 (a quantitative analysis of the above operation will be described later on).

The first and second capacitors C₁, C₂ operate with a certain DC voltage because of the operation of the booster chopper circuit shown in FIG. 12a. In the half-bridge circuit shown in FIG. 12b, the ratio of the on-time of one of the first switching element Q₁ to the operating period T₃₁ is varied.

The waveforms of voltages and currents shown in FIG. 10 will be described in detail below.

In the on-time T_(ON31) of the first switching element Q₁, the current I(L₁) flows from the input power supply V_(in) through the first choke coil L₁ to the first switching element Q₁. At this time, the current I(L₁) flows along the path indicated by J₁ in FIG. 6, and has a waveform those gradient is indicated by V_(a) /L_(a) where V_(a) represents the voltage of the input power supply V_(in) and L_(a) represents the inductance of the first choke coil L₁.

When the first capacitor C₁ is discharged, the current I(N₁) flows from the positive-voltage terminal of the first capacitor C₁ through the primary winding N₁ of the transformer T and the first switching element Q₁ back to the negative-voltage terminal (ground terminal) of the first capacitor C₁, along the path indicated by J₄ in FIG. 9. When the current I(N₁) flows through the primary winding N₁, it induces a voltage across the second secondary winding N_(b), turning on the second synchronous rectifying transistor Q₄. A voltage is then induced across the first secondary winding N_(a), causing a current to flow along a path indicated by J₆ in FIG. 9. The current supplies electric energy to the load 17, and charges the output capacitor C_(out). It is assumed that at this time the current flows through the primary winding N₁ in a positive direction.

The current flowing through the primary winding N₁ is equal to the sum of a current converted from the current flowing through the second choke current L₂ with the turns ratio of the transformer T and an magnetizing current of the transformer T.

As described above, the current which is equal to the sum of the current flowing through the first choke coil L₁ along the path indicated by J₁ in FIG. 6 and the current flowing through the primary winding N₁ along the path indicated by J₄ in FIG. 9 flows through the first switching element Q₁. This current has a waveform I(Q₁) as shown in FIG. 10.

In the on-time T_(ON32) in which the first switching element Q₁ is turned off and the second switching element Q₂ is turned on, the current I (L₁) flows through the first choke coil L₁. The current I(L₁) is generated by an electromotive force induced across the first choke coil L₁, and flows from the positive-voltage terminal of the first choke coil L₁ through the second switching element Q₂, the second capacitor C₂, the first capacitor C₁, and the input power supply V_(in) back to the negative-voltage terminal of the first choke coil L₁, along the path indicated by J₂ in FIG. 7, charging the series-connected circuit of the first and second capacitors C₁, C₂. This current corresponds to the current flowing through the first choke coil L₁ and the diode D₂₁ with the first switching element Q₁ being turned off, in the booster chopper circuit shown in FIG. 12c.

The current has a wavef orm whose gradient is indicated by (V_(a) -V_(b))/L_(a) where V_(a) represents the voltage of the input power supply V_(in), L_(a) represents the inductance of the first choke coil L₁, and V_(b) represents the voltage across the series-connected circuit of the first and second capacitors C₁, C₂.

In the on-time T_(ON32) of the second switching element Q₂, the second capacitor C₂ is discharged, causing the current I(N₁) to flow through the primary winding N₁ of the transformer T, along the path indicated by J₃ in FIG. 8. This current flows from the positive-voltage terminal of the second capacitor C₂ through the second switching element Q₂ and the primary winding N₁ back to the negative-voltage terminal of the second capacitor C₂.

At this time, the current flowing through the primary winding N₁ is equal to the sum of a current converted from the current flowing through the second choke current L₂ with the turns ratio of the transformer T and an magnetizing current of the transformer T.

Since the current flows through the primary winding N₁ along the path indicated by J₃ in FIG. 8, a positive voltage is applied to the gate terminal of the first synchronous rectifying transistor Q₃ by an electromotive force induced across the first secondary winding N_(a), thus turning on the first synchronous rectifying transistor Q₃.

As a result, due to the electromotive force developed across the second secondary winding N_(b), a current flows along a path indicated by J₅ in FIG. 8, supplying electric energy to the load 17 and charging the output capacitor C_(out).

In the on-time T_(ON32) of the second switching element Q₂, the electromotive force developed across the first choke coil L₁ causes a current to flow through the second switching element Q₂ in the direction to charge the first and second capacitors C₁, C₂ along the path indicated by J₂ in FIG. 7.

Therefore, in the on-time T_(ON32) of the second switching element Q₂, the current flows from the drain terminal to the source terminal of the second switching element Q₂ along the path indicated by J₃ in FIG. 8 because of the discharging of the second capacitor C₂, and the current flows from the source terminal to the drain terminal of the second switching element Q₂ along the path indicated by J₂ in FIG. 7 because of the electromotive force developed across the first choke coil L₁.

As a consequence, a current which is the difference between the current I(L₁) flowing through the first choke coil L₁ and the current I(N₁) flowing through the primary winding N₁ of the transformer T flows through the second switching element Q₂. This current has a waveform indicated by I(Q₂) in FIG. 10.

In FIG. 10, V(N₁) represents a voltage developed across the primary winding N₁ of the transformer T. This voltage in the on-time T_(ON31) corresponds to the voltage across the first capacitor C₁ because the first switching element Q₁ is turned on, and in the on-time T_(ON32) corresponds to the voltage across the second capacitor C₂ because the second switching element Q₂ is turned on.

In FIG. 10, V_(ds) (Q₃), V_(ds) (Q₄) represent, respectively, drain-to-source voltages of the first and second synchronous rectifying transistors Q₃, Q₄ in FIG. 5. These drain-to-source voltages serve as respective gate drive voltages of the second and first synchronous rectifying transistors Q₄, Q₃.

In FIG. 10, V(R) represents a voltage at the point R in FIG. 5 between the first and second secondary windings N_(a), N_(b) of the transformer T, and I(L₂) represents a current flowing through the second choke coil L₂.

The drain-to-source voltages V_(ds) (Q₃), V_(ds) (Q₄) are voltages converted from the voltage V(N₁) across the primary winding N₁ respectively in the on-times T_(ON31), T_(ON32) with the turns ratio between the primary winding N₁ and the first secondary winding N_(a) (or the second secondary winding N_(b)). The voltage V(R) at the point R is equal to the sum of the drain-to-source voltages V_(ds) (Q₃), V_(ds) (Q₄).

Of the voltage V(R), V_(out) represents the output voltage between the output terminals 16_(a), 16_(b). The voltage V(R) at the point R and the output voltage V_(out) are applied across the second choke coil L₂, causing a ripple current I(L₂) to flow through the second choke coil L₂. A ripple voltage whose value is substantially determined by the product of the ripple current I(L₂) and an equivalent series resistance of the output capacitor C_(out) is generated in the output voltage.

As can be seen from the above description, according to the first embodiment shown in FIG. 5, either one of the drain-to-source voltages V_(ds) (Q₃), V_(ds) (Q₄) of the first and second synchronous rectifying transistors Q₃, Q₄ is generated at all times except for the short off-times T_(off31), T_(off32).

Therefore, when a voltage is induced across the first secondary winding N_(a) or the second secondary winding N_(b), applying a higher voltage to the source terminal of the second synchronous rectifying transistor Q₄ or the second synchronous rectifying transistor Q₃ than to the drain terminal thereof, a positive voltage is applied to the gate terminal of the synchronous rectifying transistor, turning on the synchronous rectifying transistor. Therefore, the synchronous rectifying transistor is prevented from being turned off, preventing a current from flowing from the source terminal to the drain terminal thereof through the internal parasitic diode, so that no large loss will be caused. The switching power supply according to the present invention is therefore free of "the problem of a large loss because of a long period of time in which the synchronous rectifying transistor could not be energized."

As can be understood from the waveforms of the drain-to-source voltages V_(ds) (Q₃), V_(ds) (Q₄), since the voltages applied to the first and second synchronous rectifying transistors Q₃, Q₄ are of rectangular waveforms, these voltages are prevented from being unduly increased whereas they posed a problem in the conventional circuit arrangement because they are of a resonating waveform. The switching power supply according to the present invention may thus employ synchronous rectifying transistors having a low dielectric strength and a low on-state resistance.

An output voltage of the switching power supply shown in FIG. 5 with respect to a duty cycle (the ratio of the on-time to the operating period of the switching element Q₁) thereof will be described with reference to FIG. 11.

It is assumed that the voltage of the input power supply V_(in) is represented by V_(a), the duty cycles of the respective first and second switching elements Q₁, Q₂ are represented by D, 1-D, respectively, the voltages across the first and second capacitors C₁, C₂ are represented by V(C₁), V(C₂), respectively, the turns ratio between the primary winding N₁ and the first secondary winding N_(a) (or the second secondary winding N_(b)) is represented by n: 1, the drain-to-source voltages of the first and second synchronous rectifying transistors Q₃, Q₄ are represented by V_(ds) (Q₃), V_(ds) (Q₄), respectively, and the output voltage between the output terminals 16_(a), 16_(b) is represented by V_(out). From the circuit arrangement of the booster chopper circuit shown in FIG. 12a, the following equation (1) is satisfied (it is also assumed that, in the following numerical analysis, voltage drops developed across the first and second switching elements Q₁, Q₂ and the first and second synchronous rectifying transistors Q₃, Q₄ when they are rendered conductive are negligibly small, and the off-times T_(off31), T_(off32) in which both the first and second switching elements Q₁, Q₂ are turned off are also negligibly small):

    V(C.sub.1)+V(C.sub.2)=V.sub.a /(1-D)                       (1)

With respect to the operation of the core (magnetic material) of the transformer T, the quantity by which the core is excited in the on-time of the first switching element Q₁ is equal to the quantity by which the core is reset in the on-time of the second switching element Q₂. Therefore, the following equation (2) is satisfied:

    V(C.sub.1)×D=V(C.sub.2)×(1-D)                  (2)

From the equations (1), (2), the following equations (3), (4) are derived:

    V(C.sub.1)=V.sub.a                                         (3)

    V(C.sub.2)=V.sub.a ×D/(1-D)                          (4)

Since the drain-to-source voltages of the first and second synchronous rectifying transistors Q₃, Q₄, at the time they are turned off are voltages converted from the voltages across the first and second capacitors C₁, C₂ with the turns ratio of the transformer T, the following equations (5), (6) are satisfied:

    V.sub.ds (Q.sub.3)=V(C.sub.1)/n=V.sub.a /n                 (5)

    V.sub.ds (Q.sub.4)=V(C.sub.2)/n=V.sub.a ×D/{(1-D)×n}(6)

The output voltage between the output terminals 16_(a), 16_(b) is of a value produced by averaging the voltage at the point R with the output filter, and the voltage at the point R is equal to the sum of the drain-to-source voltages V_(ds) (Q₃), V_(ds) (Q₄) of the first and second synchronous rectifying transistors Q₃, Q₄. Therefore, if the switching period is represented by T₀, then the following equation (7) is satisfied: ##EQU1##

From the equation (7), it can be seen that the output voltage V_(out) of the switching power supply shown in FIG. 5 is proportional to the duty cycle D of the switching element Q₁, as shown in FIG. 11.

Since the output voltage V_(out) is proportional to the duty cycle D of the switching element Q₁, as shown in FIG. 11, it is possible to set the duty cycle D to 0.5 provided the input and output conditions are rated conditions. At this time, both the drain-to-source voltages V_(ds) (Q₃), V_(ds) (Q₄) of the first and second synchronous rectifying transistors Q₃, Q₄ are represented by V_(a) /n as can be understood from the equations (5), (6). Inasmuch as the drain-to-source voltages V_(ds) (Q₃), V_(ds) (Q₄) are variable about the value V_(a) /n while keeping the rectangular waveform even when the input and output conditions are varied, the first and second synchronous rectifying transistors Q₃, Q₄ are not required to be of large dielectric strength and large on-state resistance unlike those of the conventional switching power supply.

The fact that the drain-to-source voltages V_(ds) (Q₃), V_(ds) (Q₄) are equal to each other (actually, they are slightly different from each other as the numbers of turns of the transformer T are integers) means that any changes in the voltage V(R) at the point R are very small. Because the voltage in the on-time T_(ON31) and the voltage in the on-time T_(ON32) are equal to each other or different from each other very slightly, the second choke coil L₂ may be small, and any power loss caused by the second choke coil L₂ may be small, so that the switching power supply may be highly efficient. Actually, the output filter is designed taking into account variations in the input and output conditions, but the second choke coil L₂ may still be small.

The above analytical results have experimentally been verified.

In FIG. 10, I(N₁) represents a current flowing through the primary winding N₁ of the transformer T. Since the transformer T has a leakage inductance, the off-times T_(off31), T_(off31) in which both the first and second switching elements Q₁, Q₂ are turned off may be adjusted appropriately such that after one of the first and second switching elements Q₁, Q₂ is turned off and before the other of the first and second switching elements Q₁, Q₂ is turned on, a current due to the leakage inductance discharges the parasitic capacitor between the drain and source of the other switching element. As a result, the switching power supply can operate as a ZVS (Zero Voltage Switching) power supply.

Since the energy stored in the parasitic capacitor between the drain and source of the other switching element can be recovered, the efficiency of the switching power supply is increased.

As described above, with the switching power supply shown in FIG. 5, either one of the first and second synchronous rectifying transistors Q₃, Q₄ is energized at all times except for the short off-times T_(off31), T_(off32), i.e., the synchronous rectifying transistor through which the current flowing through the second choke coil L₂ flows is energized, and their applied drain-to-source voltage is low. Therefore, the first and second synchronous rectifying transistors Q₃, Q₄ may be of small dielectric strength and small on-state resistance, and at the same time the output filter may be small and any power loss caused by the output filter may be small.

As a consequence, the switching power supply is highly efficient in operation.

In the switching power supply according to the second embodiment shown in FIG. 5, each of the first and second switching elements Q₁, Q₂ comprises an n-channel MOSFET. However, one or both of the first and second switching elements Q₁, Q₂ may comprise a p-channel MOSFET.

Furthermore, each of the first and second switching elements Q₁, Q₂ is not limited to a MOSFET, but may comprise an IGBT.

In the switching power supply shown in FIG. 5, since the first and second switching elements Q₁, Q₂ are symmetrically positioned, the series-connected circuit of the input power supply V_(in) and the first choke coil L₁ may be connected between the drain and source of the second switching element Q₂ rather than between the drain and source of the first switching element Q₁. The switching power supply thus modified operates in the same manner as described above except that the first and second switching elements Q₁, Q₂ are switched around.

In the switching power supply shown in FIG. 5, the full-wave rectifier circuit comprising two rectifiers is connected to the secondary windings N_(a), N_(b) of the transformer T. However, a half-wave rectifier circuit with one of the two rectifiers used as a freewheeling rectifier may be connected to the secondary windings N_(a), N_(b) of the transformer T, or a full-wave rectifier circuit comprising a bridge of four rectifiers may be connected to the secondary windings N_(a), N_(b) of the transformer T.

A process of driving the first and second synchronous rectifying transistors Q₃, Q₄ in the switching power supply shown in FIG. 5 will be described below.

The gate terminal of each of the first and second synchronous rectifying transistors Q₃, Q₄ in the switching power supply shown in FIG. 5 is turned on by the drain-to-source voltage of the other first and second synchronous rectifying transistors Q₃, Q₄. However, the gate terminal of each of the first and second synchronous rectifying transistors Q₃, Q₄ may be turned on by a voltage obtained from the windings of the transformer T.

Another process of driving the first and second synchronous rectifying transistors Q₃, Q₄ is shown in FIG. 13. A switching power supply shown in FIG. 13 operates in a manner equivalent to the switching power supply shown in FIG. 5.

In the switching power supply according to the second embodiment shown in FIG. 14, the source terminal of a second switching element Q₅₂ is connected to the drain terminal of the first switching element Q₁, and the first and second switching element Q₁, Q₅₂ are connected in series with each other, thus jointly making up a series-connected circuit 44. A second capacitor C₅₂ is connected parallel to the series-connected circuit 44, jointly making up a parallel-connected circuit 45.

The switching power supply shown in FIG. 14 is equivalent to the switching power supply shown in FIG. 5, and operates in the same manner as the switching power supply shown in FIG. 5. The switching power supply shown in FIG. 14 is equivalent to the switching power supply shown in FIG. 5 for the reasons which will be described below with reference to FIGS. 15a and 15b.

FIGS. 15a and 15b show half-bridge circuits which correspond to the switching power supplies according to the first and second embodiments, respectively. In FIG. 15a, the second switching element Q₂ and the second capacitor C₂ are connected in series with each other. In FIG. 15b, the second switching element Q₅₂ and the second capacitor C₅₂ are connected in series with each other. If the capacitor C₅₂ shown in FIG. 15b has a capacitance equivalent to the combined capacitance of the series-connected capacitors C₁, C₂, then the capacitor C₅₂ shown in FIG. 15b may be replaced with a capacitor C₁₀₀ shown in FIG. 15b which has the same capacitance as the capacitance of the capacitor C₂.

Therefore, the switching power supply shown in FIG. 14 offers the same advantages as those of the switching power supply according to the second embodiment shown in FIG. 5.

FIG. 16 shows a switching power supply according to a third embodiment of the present invention. In the switching power supply shown in FIG. 16, the first choke coil L₁ has a terminal connected to the input terminal 2_(a) which is connected to the positive-voltage terminal of the input power supply V_(in). The other terminal of the first choke coil L₁ is connected to a terminal of the primary winding N₁ of a transformer T. The other terminal of the primary winding N₁ is connected by the first capacitor C₁ to the input terminal 2_(b) which is kept at ground potential.

The primary winding N₁ and the first capacitor C₁ jointly make up a series-connected circuit 47. The first switching element Q₁ which comprises an n-channel MOSFET is connected parallel to the series-connected circuit 47. The first switching element Q₁ has a source terminal connected to ground and a drain terminal connected to the junction between the first choke coil L₁ and the primary winding N₁.

The parts connected to the first and second secondary windings N_(a), N_(b) of the transformer T are identical to those of the switching power supply according to the first embodiment. The first and second secondary windings N_(a), N_(b) are magnetically coupled to the primary winding N₁ of the transformer T. The switching power supply shown in FIG. 16 additionally has a tertiary winding N₃ magnetically coupled to the primary winding N₁ and the first and second secondary windings N_(a), N_(b).

The tertiary winding N₃ has a terminal connected by a second capacitor C₆₂ to the drain terminal of a second switching element Q₆₂ which comprises an n-channel MOSFET. The other terminal of the tertiary winding N₃ is connected to the source terminal of the second switching element Q₆₂.

The tertiary winding N₃ and the second capacitor C₆₂ jointly make up a series-connected circuit 48. The second switching element Q₆₂ is connected parallel to the series-connected circuit 48.

The terminal of the tertiary winding N₃ which is connected to the second switching element Q₆₂ is of the same polarity as that of the terminal of the primary winding N₁ which is connected to the drain terminal of the first switching element Q₁.

Operation of the switching power supply shown in FIG. 16 will be described below. After a voltage from the input power supply V_(in) is applied between the input terminals 2_(a), 2_(b), when the first switching element Q₁ is turned on with the second switching element Q₆₂ being turned off, electric energy is supplied from the input power supply V_(in), causing a current to flow through the first choke coil L₁ to the first switching element Q₁ along a path indicated by J₁₁ in FIG. 17.

When the first switching element Q₁ is then turned off, a current flows along a path indicated by J₁₂ in FIG. 18 due to an electromotive force developed across the first choke coil L₁.

The current starts to flow from the terminal of the first choke coil L₁ where a positive voltage is induced, and flows through the primary winding N₁, thereby inducing voltages across the respective first and second secondary windings N_(a), N_(b). The current flowing through the primary winding N₁ flows into the first capacitor C₁, thus charging first capacitor C₁. The current thus passes through the input power supply V_(in), and flows back to the terminal of the first choke coil L₁ where a negative voltage is induced.

After the current has started to flow along the path indicated by J₁₂ in FIG. 18 (at this time, the first switching element Q₁ has changed from the on-state to the off-state), when the second switching element Q₆₂ is turned on, a current flows from the source terminal to drain terminal of the second switching element Q₆₂ along a path indicated by J₁₄ in FIG. 18 due to an electromotive force developed across the tertiary winding N₃, charging the second capacitor C₆₂.

The first and second capacitors C₁, C₆₂ are thus charged when the first and second switching element Q₁, Q₆₂ are alternately rendered conductive.

The state in which the first and second capacitors C₁, C₆₂ are charged is referred to as a steady state. When the second switching element Q₆₂ is rendered conductive with the second capacitor C₆₂ being charged, a current flows along a path indicated by J₁₅ in FIG. 18 due to the discharging of the second capacitor C₆₂ at the same time that the current flows along the path indicated by J₁₄ . The current flowing along the path indicated by J₁₅ is opposite in direction to the current flowing along the path indicated by J₁₄, and these current cancel out each other.

At this time, voltages are induced respective across the first and second secondary windings N_(a), N_(b) by the current flowing through the primary winding N₁. The induced voltages are of such polarity that the second synchronous rectifying transistor Q₄ is turned on, and a current flows from the terminal of the first secondary windings N_(a) where a positive voltage is induced, through the second choke coil L₂, the output capacitor C_(out) (or the load), the source terminal of the second synchronous rectifying transistor Q₄, and the drain terminal thereof, to the terminal of the first secondary windings N_(a) where a negative voltage is induced, along a path indicated by J₁₃ in FIG. 18.

When the second switching element Q₆₂ is turned off and the first switching element Q₁ is turned on, a current is supplied from the input power supply V_(in) along the path indicated by J₁₁ in FIG. 17, and the first capacitor C₁ is discharged along a path indicated by J₁₆ in FIG. 19.

Because of the discharging of the first capacitor C₁, a current flows through the primary winding N₁, turning on the first synchronous rectifying transistor Q₃, and a current flows along a path indicated by J₁₇ in FIG. 19.

When the first and second switching elements Q₁, Q₆₂ are thus alternately rendered conductive, the first and second synchronous rectifying transistors Q₃, Q₄ are also alternately rendered conductive, supplying electric energy to the output capacitor C_(out) and the load. While voltages are being induced across the first and second secondary windings N_(a), N_(b), the first synchronous rectifying transistor Q₃ or the second synchronous rectifying transistor Q₄ is rendered conductive, with no current flowing through the internal parasitic diode. This operation is the same as the operation of the switching power supplies according to the first and second embodiments.

In the switching power supply according to the third embodiment shown in FIG. 16, voltages can be converted from the secondary side to the primary side of the transformer T with the turns ratio of the transformer T. The switching power supply according to the third embodiment has an equivalent circuit which is the same as the equivalent circuit of the switching power supply according to the first embodiment shown in FIG. 5. The switching power supply according to the third embodiment shown in FIG. 16 offers the same advantages as those of the switching power supply according to the first embodiment shown in FIG. 5.

According to the present invention, either one of the first and second synchronous rectifying transistors Q₃, Q₄ is energized at all times except for the short off-times T_(off31), T_(off32). Therefore, the first and second synchronous rectifying transistors Q₃, Q₄ may be of small dielectric strength and small on-state resistance, and at the same time the output filter may be small. Consequently, the switching power supply may be highly efficient in operation.

It is thus possible according to the present invention to make a highly efficient switching power supply with a low-voltage (e.g., 5 V or 3.3 V), high-current output capability for use in the field of communications or the like.

Although certain preferred embodiments of the present invention have been shown and described in detail, it should be understood that various changes and modifications may be made therein without departing from the scope of the appended claims. 

What is claimed is:
 1. A power supply circuit comprising:first and second choke coils; first and second capacitors; first and second switching elements each comprising a MOSFET; first and second synchronous rectifying transistors each comprising a MOSFET; a transformer having a primary winding and first and second secondary windings which are magnetically coupled to said primary winding; said first and second switching elements being connected in series with each other at a junction, jointly making up a series-connected circuit, said primary winding and said first choke coil having respective terminals connected to said junction between the first and second switching elements; said first and second capacitors being connected in series with each other at a junction, jointly making up a series-connected circuit, said primary winding having another terminal connected to said junction between the first and second capacitors; said series-connected circuit of the first and second switching elements and said series-connected circuit of the first and second capacitors being connected parallel to each other, jointly making up a parallel-connected circuit; and a power supply, operably connected to a terminal of said parallel-connected circuit and another terminal of said first choke coil, for applying a voltage between said terminal of said parallel-connected circuit and said another terminal of said first choke coil wherein when said first and second switching elements are alternately tuned on, an alternating current flows through said primary winding to induce alternating-current voltages respectively across said first and second secondary windings for thereby alternately turning on said first and second synchronous rectifying transistors to cause a current to flow alternatively through said first and second secondary windings for supplying a current to said second choke coil.
 2. A power supply circuit comprising:first and second choke coils; first and second capacitors; first and second switching elements each comprising a MOSFET; first and second synchronous rectifying transistors each comprising a MOSFET; a transformer having a primary winding and first and second secondary windings which are magnetically coupled to said primary winding; said first and second switching elements being connected in series with each other at a junction, jointly making up a series-connected circuit, said primary winding and said first choke coil having respective terminals connected to said junction between the first and second switching elements; said second capacitor being connected parallel to said series-connected circuit of the first and second switching elements, jointly making up a parallel-connected circuit; said parallel-connected circuit having a terminal connected by said first capacitor to another terminal of said primary winding; and a power supply, operably connected to said parallel-connected circuit, said first capacitor, and another terminal of said first choke coil, for applying a voltage between a junction between said parallel-connected circuit and said first capacitor, and said another terminal of said first choke coil, wherein when said first and second switching elements are alternately turned on, an alternating current flows through said primary winding to induce alternating-current voltages respectively across said first and second secondary windings for thereby alternately turning on said first and second synchronous rectifying transistors to cause a current to flow alternatively through said first and second secondary windings for supplying a current to said second choke coil.
 3. A power supply circuit comprising:first and second choke coils; first and second capacitors; first and second switching elements each comprising a MOSFET; first and second synchronous rectifying transistors each comprising a MOSFET; a transformer having a primary winding, first and second secondary windings which are magnetically coupled to said primary winding, and a tertiary winding which is magnetically coupled to said primary winding and said first and second secondary windings; said primary winding and said first capacitor being connected in series with each other, jointly making up a series-connected circuit, said first switching element being connected parallel to said series-connected circuit of the primary winding and the first capacitor, jointly making up a parallel-connected circuit; said parallel-connected circuit having a terminal connected to a terminal of said first choke coil; and a power supply, operably connected to another terminal of said parallel-connected circuit and another terminal of said first choke coil, for applying a voltage between said another terminal of said parallel-connected circuit and said another terminal of said first choke coil; said tertiary winding and said second capacitor being connected in series with each other, jointly making up a series-connected circuit, said second switching element being connected parallel to said series-connected circuit of the tertiary winding and the second capacitor, wherein when said first and second switching elements are alternately turned on, an alternating current flows through said primary winding to induce alternating-current voltages respectively across said first and second secondary windings for thereby alternately turning on said first and second synchronous rectifying transistors to cause a current to flow alternately through said first and second secondary windings for supplying a current to said second choke coil.
 4. A power supply circuit according to any one of claims 1 through 3, wherein said first and second secondary windings have respective terminals connected to each other at a junction, said second choke coil having a terminal connected to said junction between said first and second secondary windings, and wherein said first and second secondary windings have respective other terminals connected to each other by said first or second synchronous rectifying transistor at a junction, with an output voltage being extracted between the junction between the other terminals of the first and second secondary windings and another terminal of said second choke coil, the arrangement being such that a voltage at the other terminal of the second secondary winding is applied to a gate terminal of one of said first and second synchronous rectifying transistors which is connected to said first secondary winding, and a voltage at the other terminal of the first secondary winding is applied to a gate terminal of one of said first and second synchronous rectifying transistors which is connected to said second secondary winding.
 5. A power supply circuit according to claim 4, wherein either one of said first and second switching elements is turned on for an on-time except for a relatively short off-time in which both of said first and second switching elements are prevented from being turned on at the same time, said on-time being variable to keep constant the output voltage extracted from said second choke coil.
 6. A power supply circuit according to claim 5, further comprising an output capacitor for smoothing said output voltage. 